Nylon and other conventional polyamide resins are used throughout the magnet wire industry for the purpose of insulating the wire, either as a solecoat construction (the only insulating layer) or a topcoat applied over a basecoat enamel, e.g. polyester, polyurethane, etc. In using such polyamide resins in this way, a number of problems arise during their application. In particular, it is noted that the available polyamide resins have very high molecular weights, typically ranging from about 18,000 to about 40,000 molecular weight units (weight average). These high molecular weight materials mean that a resulting solution of this material at a high solids content (greater than 17 percent by weight of solids) can have a viscosity at room temperature (about 77.degree. F.) of about 4,000 centipoises to about 11,000 centipoises. Since the typical enamel or coating which may be easily applied to magnet wire has a viscosity of about 50 centipoises to about 2000 centipoises at application temperature greater than 100.degree. F. (37.8.degree. C.). The ability to prepare an acceptable high solids enamel using this nylon is not practical. These polyamide solutions are often prepared having very low solids contents of about 5 percent to about 16 percent by weight of solids. This lowers the viscosity to a level which is within the working range of the magnet wire enamel applicators.
Unfortunately, this reduction in solids content means that more diluent must be used resulting in an added cost to production and an increase in environmental pollutants. In addition, the lower solids content can mean more passes for the wire through the coating are required to apply the desired coating thickness onto the wire, again adding cost to the wire production.
It is known that the molecular weight and thereby the viscosity of the polyamide resin coatings may be reduced by equilibrating the resin. Equilibration, as used in this application, refers to the solvolytic molecular weight reduction of a polymer. A common equilibration process which is used requires the reaction of monoethanolamine with the polyamide resin as shown in reaction 1 below: ##STR1## This reaction requires that an excess of monoethanolamine be present during the reaction. The excess must be neutralized prior to the coating being applied to the wire and is accomplished by adding acetic anhydride to the solution which reacts with the monoethanolamine in the primary reaction as shown in reaction 2 below: ##STR2## However certain additional side reactions also occur during this neutralization process, the most important of which is shown below in reaction 3. ##STR3## This reaction being reversible at relatively low temperatures (about 80.degree. C. to about 100.degree. C.) means that during the oven curing of the coating (which temperatures exceed those to reverse the reaction) acetic acid and monoethanolamine are produced. The acetic acid is easily volatilized out of the coating during curing but the ethanolamine is not completely removed and remains behind. This material is known to be deleterious to many wire varnish curing mechanisms. Therefore, the use of this nylon as a wire coating (particularly for fine wires) results in wire having undercured coatings which do not effectively adhere to the wire or to wire whose mechanical and electrical properties are prematurely reduced during use.
In addition, the resulting wire coatings do not exhibit uniform properties from batch to batch and quality control is difficult. One evidence of this is the fluctuation in the mandrel pull test used to test the ability of wire coating to withstand numerous cycles of bending about a mandrel without exhibiting cracks or defects in the wire coating. Coatings formed using the conventional monoethanolamine approach to preparing the equilibrated polyamide exhibits wide fluctuations in its ability to pass such tests and in many instances the wire is unacceptable.
Therefore, what is needed in the art is a method by which the polyamide resin may be equilibrated without the introduction or resulting byproducts which will affect the quality of the final coated wire product.